STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (USD $) In Thousands, except Share data	Total	Employee Stock Option	Common Stock	Common Stock Employee Stock Option	Additional Paid-in Capital Common Stock	Additional Paid-in Capital Common Stock Employee Stock Option	Additional Paid-in Capital Warrants	Deficit Accumulated During the Development Stage	Registered direct offering	Registered direct offering Common Stock	Registered direct offering Additional Paid-in Capital Common Stock	Collaboration Agreement	Collaboration Agreement Common Stock	Collaboration Agreement Additional Paid-in Capital Common Stock
Beginning Balance at Dec. 31, 2011	$ 71,607		$ 69		$ 246,519		$ 12,611	$ (187,592)
Beginning Balance (in shares) at Dec. 31, 2011			69,206,044
Stock-based compensation	4,880				4,880
Exercise of warrants to purchase common stock (in shares)	259,660		259,660
Issuance of common stock (in shares)										10,114,401			3,636,926
Issuance of common stock									49,170	11	49,159	18,694	3	18,691
Exercise of warrants to purchase common stock	742				1,011		(269)
Exercise of employee stock options (in shares)	8,300			8,300
Exercise of employee stock options		30				30
Issuance of restricted common stock			258,032
Repurchase of shares of restricted common stock (in shares)			(123,153)
Repurchase of shares of restricted common stock	(546)				(546)
Cancelled/ Forfeiture of unvested restricted common stock			(123,370)
Expired warrants					5,433		(5,433)
Net loss	(96,132)							(96,132)
Ending Balance at Dec. 31, 2012	48,445		83		325,177		6,909	(283,724)
Ending Balance (in shares) at Dec. 31, 2012			83,236,840
Stock-based compensation	3,507				3,507
Exercise of warrants to purchase common stock (in shares)	112,808		112,808
Issuance of common stock (in shares)										16,445,000
Issuance of common stock									53,880	16	53,864
Exercise of warrants to purchase common stock	200				396		(196)
Exercise of employee stock options (in shares)	570,168			570,168
Exercise of employee stock options		956		1		955
Issuance of restricted common stock			75,272
Repurchase of shares of restricted common stock (in shares)			(116,723)
Repurchase of shares of restricted common stock	(498)				(498)
Cancelled/ Forfeiture of unvested restricted common stock			(163,747)
Expired warrants					3,110		(3,110)
Net loss	(57,107)							(57,107)
Ending Balance at Dec. 31, 2013	49,383		100		386,511		3,603	(340,831)
Ending Balance (in shares) at Dec. 31, 2013			100,159,618
Stock-based compensation	4,743				4,743
Exercise of warrants to purchase common stock (in shares)	3,725,277		3,747,254
Exercise of warrants to purchase common stock	10,654		4		13,963		(3,313)
Exercise of employee stock options (in shares)	613,138			613,138
Exercise of employee stock options		1,386				1,386
Issuance of restricted common stock			66,828
Repurchase of shares of restricted common stock (in shares)			(112,333)
Repurchase of shares of restricted common stock	(544)				(544)
Cancelled/ Forfeiture of unvested restricted common stock			(22,400)
Expired warrants					290		(290)
Net loss	(31,781)							(31,781)
Ending Balance at Dec. 31, 2014	$ 33,841		$ 104		$ 406,349		$ 0	$ (372,612)
Ending Balance (in shares) at Dec. 31, 2014			104,452,105